Method and composition for crystallizing G protein-coupled receptors

ABSTRACT

Certain embodiments provide a method for crystallizing a GPCR. The method may employ a fusion protein comprising: a) a first portion of a G-protein coupled receptor (GPCR), where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertion; and c) a second portion of the GPCR, where the second portion comprises the TM6 and TM7 regions of the GPCR.

GOVERNMENT RIGHTS

This invention was made with Government support under contract NS028471 awarded by the National Institutes of Health. The Government has certain rights in this invention.

BACKGROUND

G protein-coupled receptor (GPCR) signaling plays a vital role in a number of physiological contexts including, but not limited to, metabolism, inflammation, neuronal function, and cardiovascular function. For instance, GPCRs include receptors for biogenic amines, e.g., dopamine, epinephrine, histamine, glutamate, acetylcholine, and serotonin; for purines such as ADP and ATP; for the vitamin niacin; for lipid mediators of inflammation such as prostaglandins, lipoxins, platelet activating factor, and leukotrienes; for peptide hormones such as calcitonin, follicle stimulating hormone, gonadotropin releasing hormone, ghrelin, motilin, neurokinin, and oxytocin; for non-hormone peptides such as beta-endorphin, dynorphin A, Leu-enkephalin, and Met-enkephalin; for the non-peptide hormone melatonin; for polypeptides such as C5a anaphylatoxin and chemokines; for proteases such as thrombin, trypsin, and factor Xa; and for sensory signal mediators, e.g., retinal photopigments and olfactory stimulatory molecules.

GPCRs are of immense interest for drug development.

SUMMARY OF THE INVENTION

A fusion protein is provided. In certain embodiments, the fusion protein comprises: a) a first portion of a G-protein coupled receptor (GPCR), where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertion, e.g., the amino acid sequence of lysozyme; and c) a second portion of the GPCR, where the second portion comprises the TM6 and TM7 regions of the GPCR. The polypeptide may be employed in crystallization methods, for example.

In certain embodiments, the stable, folded protein insertion is a polypeptide than can fold autonomously in a variety of cellular expression hosts, and is resistant to chemical and thermal denaturation. In particular embodiments, the stable folded protein insertion may be a protein that is known to be highly crystallizable, in a variety of space groups and crystal packing arrangements. In certain cases, the stable, folded protein insertion may also shield the fusion protein from proteolysis between the TM5 and TM6 domains, and may itself be protease resistant. Lysozyme is one such polypeptide, however many others are known.

Also provided is a nucleic acid encoding the above described fusion protein, and a cell comprising the same. The fusion protein may be disposed on the plasma membrane of the cell.

Also provided are crystals comprising the above described fusion protein, folded into an active form.

The above-described cell may be employed in a method comprising: culturing the cell to produce the fusion protein; and isolating said fusion protein from the cell. The method may further comprise crystallizing the fusion protein to make crystals which, in certain embodiments, may involve combining the fusion protein with lipid prior to crystallization. In certain embodiments, the fusion protein is crystallized using a bicelle crystallization method or a lipidic cubic phase crystallization method. The method may further comprise obtaining atomic coordinates of the fusion protein from the crystal.

Also provided is a method of determining a crystal structure. This method may comprise receiving an above described fusion protein, crystallizing the fusion protein to produce a crystal; and obtaining atomic coordinates of the fusion protein from said crystals. In other embodiments, the method may comprise forwarding a fusion protein to a remote location where the protein may be crystallized and analyzed, and receiving the atomic coordinates of the fusion protein.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a schematic illustration of a GPCR, showing the canonical transmembrane regions (TM1, TM2, TM3, TM4, TM5, TM6, and TM7), intracellular regions (IC1, IC2, and IC3), and extracellular regions (EC1, EC2, and EC3).

FIG. 2 is a schematic illustration of a subject fusion protein, showing a stable, folded protein insertion between the TM5 and TM6 regions of a GPCR.

FIG. 3 Design and optimization of the β₂AR-T4L (β₂-adrenergic receptor T4 lysozyme) fusion protein. A. The sequence of the region of the β₂AR (β₂-adrenergic receptor) targeted for insertion of a crystallizable domain is shown, and the positions of the junctions between the receptor and T4L (T4 lysozome; in red) for various constructs are indicated. The sequences that were initially replaced or removed are faded. Red lines are shown after every tenth residue. B. Immunofluorescence images of HEK293 cells expressing selected fusion constructs. Panels on the left shows M1 anti-FLAG signal corresponding to antibody bound to the N-terminus of the receptor. Panels on the right show the same signal merged with blue emission from DAPI (nuclear staining for all cells). Plasma membrane staining is observed in the positive control, D3 and D1, while C3 and D5 are retained in the endoplasmic reticulum.

FIG. 4 Functional characterization of β₂AR-T4L. A. Affinity competition curves for adrenergic ligands binding to β₂AR-T4L and wild-type β₂AR. Binding experiments on membranes isolated from Sf9 insect cells expressing the receptors were performed as described below. B. β₂AR-T4L is still able to undergo ligand-induced conformational changes. Bimane fluorescence spectra (excitation at 350 nm) of detergent-solubilized β₂AR-T4L and wild-type β₂AR truncated at 365, labeled under conditions that selectively modify Cys265^(6.27), were measured after incubating unliganded receptor with compounds for 15 min at room temperature. The cartoon illustrates that the observed changes in fluorescence can be interpreted as a movement of the bimane probe from a more buried, hydrophobic environment to a more polar, solvent-exposed position.

FIG. 5. A. Side-by-side comparison of the crystal structures of the β₂AR-T4L fusion protein and the complex between β₂AR365 and a Fab fragment. The receptor component of the fusion protein is shown as a blue cartoon (with modeled carazolol as red spheres), while the receptor bound to Fab5 is in yellow. B. Differences in the environment surrounding Phe264^(6.26) (shown as spheres) for the two proteins. C. The analogous interactions to the “ionic lock” between the E(D)RY motif and Glu247^(6.30) seen in rhodopsin (right panel, purple) are broken in both structures of the β₂AR (left panel, colored blue and yellow as above). Pymol was used for the preparation of all figures.

FIG. 6. Schematic representation of the interactions between β₂AR-T4L and carazolol at the ligand binding pocket. Residues shown have at least one atom within 4 Å of the ligand in the 2.4 Å resolution crystal structure.

FIG. 7. The ligand binding pocket of β₂AR-T4L with carazolol bound. A. Residues within 4 Å of the ligand are shown as sticks, with the exception of A200, N293, F289, and Y308. Residues that form polar contacts with the ligand (distance cutoff 3.5 Å) are in green, other residues are gray (in all panels, oxygens are colored red and nitrogens are blue). B. Same as panel A, except that the ligand is oriented with its amine facing out of the page. W109 is not shown. C. Packing interactions between carazolol and all residues within 5 Å of the ligand. View is from the extracellular side of the membrane. Carazolol is shown as yellow spheres, receptor residues are shown as sticks within van der Waals dot surfaces. Val114^(3.33), Phe193^(5.32), and Phe290^(6.52) are colored red, all other residues are gray. D. Model of (−)-isoproterenol (magenta sticks) in the ligand binding pocket observed in the crystal structure. A model of the agonist with optimal bond lengths and angles was obtained from the PRODRG server, and the dihedral angles were adjusted to the values observed in the homologous atoms of bound carazolol (16-22 in FIG. 6). The one remaining unaccounted dihedral in (−)-isoproterenol was adjusted in order to place the catechol ring in the same plane as the C₁₆-C₁₅—O₁₄ plane in carazolol. Residues known to specifically interact with agonists are shown as green sticks.

FIG. 8. Packing interactions in the β₂AR that are likely to be modulated during the activation process. A. On the left, residues previously demonstrated to be CAMs or UCMs are shown as van der Waals spheres mapped onto a backbone cartoon of the β₂AR-T4L structure. On the right, residues that are found within 4 Å of the CAMs Leu124^(3.43) and Leu272^(6.34) are shown as yellow spheres or dot surfaces. A vertical cross-section through the structure illustrates that these surrounding residues connect the CAMs on helices III and VI with the UCMs on helix VII through packing interactions. B. In both β₂AR-T4L (blue) and rhodopsin (purple), a network of ordered water molecules is found at the interface between the transmembrane helices at their cytoplasmic ends. C. Network of hydrogen bonding interactions between water molecules and β₂AR-T4L residues (sidechains as blue sticks), notably the UCMs on helix VII (orange cartoon).

FIG. 9A-9M shows the amino acid and nucleotide sequences of exemplary lysozyme fusion proteins.

FIG. 10. Affinity curves for adrenergic ligands binding to β₂AR-T4L and wild-type β₂AR. Saturation curves for the antagonist [³H]DHA is shown at left, next to competition binding curves for the natural ligand (−)-Epinephrine and the high-affinity synthetic agonist Formoterol. Binding experiments on membranes isolated from Sf9 insect cells expressing the receptors were performed as described above.

FIG. 11. Comparison of the proteolytic stability between the wild-type β₂AR and β₂AR-T4L in a limited trypsin proteolysis assay. TPCK-trypsin was added to carazolol-bound, purified, dodecylmaltoside-solubilized receptor at a 1:1000 ratio (wt:wt), and samples were analyzed by SDS-PAGE. Intact β₂AR-T4L (56.7 kD) and FLAG-tagged wild-type β₂AR (47.4 kD) migrate similarly as ˜55 kD bands. Markers are Biorad low-range SDS-PAGE protein standards.

FIG. 12. Stability comparison of unliganded β₂AR365 and β₂AR-T4L. For dodecylmaltoside-solubilized receptor preparations, maintenance of the ability to specifically bind [³H]DHA after incubation at 37° C. is taken as a measure of stability.

FIG. 13. Superimposed Cα traces of the receptor component of β₂AR-T4L (in blue) and β₂AR365 (in yellow). Common modeled transmembrane helix regions 41-58, 67-87, 108-137, 147-164, 204-230, 267-291, 312-326, 332-339 were used in the superposition by the program Lsqkab (RMSD=0.8 Å).

FIG. 14. Carazolol dissociation from β₂AR365. Dodecylmaltoside-solubilized carazolol-bound receptor (at 50 μM) was dialyzed in a large volume of buffer containing 300 micromolar alprenonol as a competing ligand, and aliquots were removed from the dialysis cassette at different time points. Remaining bound carazolol was measured (in a relative sense) by collecting fluorescence emission with excitation at 330 nm and emission from 335 to 400 nm. For each carazolol fluorescence measurement, data was normalized for the protein concentration in the dialysis cassette (measured with the Bio-Rad Protein DC kit). The Y-axis represents carazolol fluorescence emission Intensity (in cps) at 341 nm. The exponential decay of carazolol concentration in the receptor dialysis cassette was fit using Graphpad Prism software, giving a half-life of 30.4 hrs.

FIG. 15. After aligning the β₁ and β₂AR sequences, positions that have different amino acids between the two receptors were mapped onto the high-resolution structure of β₂AR-T4L (shown as red sticks). The carazolol ligand is shown as green sticks (with nitrogens in blue and oxygens in red). Highlighted residues Ala85^(2.56), Ala92^(2.63) and Tyr308^(7.35) are homologous to amino acids Leu110^(2.56), Thr117^(2.63) and Phe359^(7.35) of the β₁AR, which were shown to be primarily responsible for its selectivity over β₂AR for the compound RO363. In the β₂AR-T4L structure, only Tyr308^(7.35) faces the ligand, while Ala85^(2.56) lies at the interface between helices II and III. Of all the divergent amino acids, only Tyr308^(7.35) is found within 4 Å of any atom of carazolol.

FIG. 16 shows exemplary sequences that may be employed in place of the lysozyme sequences of FIG. 9.

DEFINITIONS

Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Singleton, et al., DICTIONARY OF MICROBIOLOGY AND MOLECULAR BIOLOGY, 2D ED., John Wiley and Sons, New York (1994), and Hale & Marham, THE HARPER COLLINS DICTIONARY OF BIOLOGY, Harper Perennial, NY (1991) provide one of skill with general dictionaries of many of the terms used in this disclosure. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described.

All patents and publications, including all sequences disclosed within such patents and publications, referred to herein are expressly incorporated by reference.

Numeric ranges are inclusive of the numbers defining the range. Unless otherwise indicated, nucleic acids are written left to right in 5′ to 3′ orientation; amino acid sequences are written left to right in amino to carboxy orientation, respectively.

The headings provided herein are not limitations of the various aspects or embodiments of the invention which can be had by reference to the specification as a whole. Accordingly, the terms defined immediately below are more fully defined by reference to the specification as a whole.

“G-protein coupled receptors”, or “GPCRs” are polypeptides that share a common structural motif, having seven regions of between 22 to 24 hydrophobic amino acids that form seven alpha helices, each of which spans a membrane. As illustrated in FIG. 1, each span is identified by number, i.e., transmembrane-1 (TM1), transmembrane-2 (TM2), etc. The transmembrane helices are joined by regions of amino acids between transmembrane-2 and transmembrane-3, transmembrane-4 and transmembrane-5, and transmembrane-6 and transmembrane-7 on the exterior, or “extracellular” side, of the cell membrane, referred to as “extracellular” regions 1, 2 and 3 (EC1, EC2 and EC3), respectively. The transmembrane helices are also joined by regions of amino acids between transmembrane-1 and transmembrane-2, transmembrane-3 and transmembrane-4, and transmembrane-5 and transmembrane-6 on the interior, or “intracellular” side, of the cell membrane, referred to as “intracellular” regions 1, 2 and 3 (IC1, IC2 and IC3), respectively. The “carboxy” (“C”) terminus of the receptor lies in the intracellular space within the cell, and the “amino” (“N”) terminus of the receptor lies in the extracellular space outside of the cell. GPCR structure and classification is generally well known in the art, and further discussion of GPCRs may be found in Probst, DNA Cell Biol. 1992 11:1-20; Marchese et al Genomics 23: 609-618, 1994; and the following books: Jürgen Wess (Ed) Structure-Function Analysis of G Protein-Coupled Receptors published by Wiley-Liss (1st edition; Oct. 15, 1999); Kevin R. Lynch (Ed) Identification and Expression of G Protein-Coupled Receptors published by John Wiley & Sons (March 1998) and Tatsuya Haga (Ed), G Protein-Coupled Receptors, published by CRC Press (Sep. 24, 1999); and Steve Watson (Ed) G-Protein Linked Receptor Factsbook, published by Academic Press (1st edition; 1994). A schematic representation of a typical GPCR is shown in FIG. 1.

The term “naturally-occurring” in reference to a GPCR means a GPCR that is naturally produced (for example and not limitation, by a mammal or by a human). Such GPCRs are found in nature. The term “non-naturally occurring” in reference to a GPCR means a GPCR that is not naturally-occurring. Wild-type GPCRs that have been made constitutively active through mutation, and variants of naturally-occurring GPCRs, e.g., epitope-tagged GPCR and GPCRs lacking their native N-terminus are examples of non-naturally occurring GPCRs.

The term “ligand” means a molecule that specifically binds to a GPCR. A ligand may be, for example a polypeptide, a lipid, a small molecule, an antibody. A “native ligand” is a ligand that is an endogenous, natural ligand for a native GPCR. A ligand may be a GPCR “antagonist”, “agonist”, “partial agonist” or “inverse agonist”, or the like.

A “modulator” is a ligand that increases or decreases a GPCR intracellular response when it is in contact with, e.g., binds, to a GPCR that is expressed in a cell. This term includes agonists, including partial agonists and inverse agonists, and antagonists.

A “deletion” is defined as a change in either amino acid or nucleotide sequence in which one or more amino acid or nucleotide residues, respectively, are absent as compared to an amino acid sequence or nucleotide sequence of a parental GPCR polypeptide or nucleic acid. In the context of a GPCR or a fragment thereof, a deletion can involve deletion of about 2, about 5, about 10, up to about 20, up to about 30 or up to about 50 or more amino acids. A GPCR or a fragment thereof may contain more than one deletion.

An “insertion” or “addition” is that change in an amino acid or nucleotide sequence which has resulted in the addition of one or more amino acid or nucleotide residues, respectively, as compared to an amino acid sequence or nucleotide sequence of a parental GPCR. “Insertion” generally refers to addition to one or more amino acid residues within an amino acid sequence of a polypeptide, while “addition” can be an insertion or refer to amino acid residues added at an N- or C-terminus, or both termini. In the context of a GPCR or fragment thereof, an insertion or addition is usually of about 1, about 3, about 5, about 10, up to about 20, up to about 30 or up to about 50 or more amino acids. A GPCR or fragment thereof may contain more than one insertion. Reference to particular GPCR or group of GPCRs by name, e.g., reference to the serotonin or histamine receptor, is intended to refer to the wild type receptor as well as active variants of that receptor that can bind to the same ligand as the wild type receptor and/or transduce a signal in the same way as the wild type receptor.

A “substitution” results from the replacement of one or more amino acids or nucleotides by different amino acids or nucleotides, respectively as compared to an amino acid sequence or nucleotide sequence of a parental GPCR or a fragment thereof. It is understood that a GPCR or a fragment thereof may have conservative amino acid substitutions which have substantially no effect on GPCR activity. By conservative substitutions is intended combinations such as gly, ala; val, ile, leu; asp, glu; asn, gln; ser, thr; lys, arg; and phe, tyr.

The term “biologically active”, with respect to a GPCR, refers to a GPCR having a biochemical function (e.g., a binding function, a signal transduction function, or an ability to change conformation as a result of ligand binding) of a naturally occurring GPCR.

As used herein, the terms “determining,” “measuring,” “assessing,” and “assaying” are used interchangeably and include both quantitative and qualitative determinations. Reference to an “amount” of a GPCR in these contexts is not intended to require quantitative assessment, and may be either qualitative or quantitative, unless specifically indicated otherwise.

The terms “polypeptide” and “protein”, used interchangeably herein, refer to a polymeric form of amino acids of any length, which can include coded and non-coded amino acids, chemically or biochemically modified or derivatized amino acids, and polypeptides having modified peptide backbones.

The term “fusion protein” or grammatical equivalents thereof is meant a protein composed of a plurality of polypeptide components, that while typically unjoined in their native state, are joined by their respective amino and carboxyl termini through a peptide linkage to form a single continuous polypeptide. Fusion proteins may be a combination of two, three or even four or more different proteins. The term polypeptide includes fusion proteins, including, but not limited to, fusion proteins with a heterologous amino acid sequence, fusions with heterologous and homologous leader sequences, with or without N-terminal methionine residues; immunologically tagged proteins; fusion proteins with detectable fusion partners, e.g., fusion proteins including as a fusion partner a fluorescent protein, β-galactosidase, luciferase, etc.; and the like.

The terms “nucleic acid molecule” and “polynucleotide” are used interchangeably and refer to a polymeric form of nucleotides of any length, either deoxyribonucleotides or ribonucleotides, or analogs thereof. Polynucleotides may have any three-dimensional structure, and may perform any function, known or unknown. Non-limiting examples of polynucleotides include a gene, a gene fragment, exons, introns, messenger RNA (mRNA), transfer RNA, ribosomal RNA, ribozymes, cDNA, recombinant polynucleotides, branched polynucleotides, plasmids, vectors, isolated DNA of any sequence, control regions, isolated RNA of any sequence, nucleic acid probes, and primers. The nucleic acid molecule may be linear or circular.

As used herein the term “isolated,” when used in the context of an isolated compound, refers to a compound of interest that is in an environment different from that in which the compound naturally occurs. “Isolated” is meant to include compounds that are within samples that are substantially enriched for the compound of interest and/or in which the compound of interest is partially or substantially purified.

As used herein, the term “substantially pure” refers to a compound that is removed from its natural environment and is at least 60% free, at least 75% free, or at least 90% free from other components with which it is naturally associated.

A “coding sequence” or a sequence that “encodes” a selected polypeptide, is a nucleic acid molecule which can be transcribed (in the case of DNA) and translated (in the case of mRNA) into a polypeptide, for example, in a host cell when placed under the control of appropriate regulatory sequences (or “control elements”). The boundaries of the coding sequence are typically determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxy) terminus. A coding sequence can include, but is not limited to, cDNA from viral, prokaryotic or eukaryotic mRNA, genomic DNA sequences from viral or prokaryotic DNA, and synthetic DNA sequences. A transcription termination sequence may be located 3′ to the coding sequence. Other “control elements” may also be associated with a coding sequence. A DNA sequence encoding a polypeptide can be optimized for expression in a selected cell by using the codons preferred by the selected cell to represent the DNA copy of the desired polypeptide coding sequence.

“Operably linked” refers to an arrangement of elements wherein the components so described are configured so as to perform their usual function. In the case of a promoter, a promoter that is operably linked to a coding sequence will effect the expression of a coding sequence. The promoter or other control elements need not be contiguous with the coding sequence, so long as they function to direct the expression thereof. For example, intervening untranslated yet transcribed sequences can be present between the promoter sequence and the coding sequence and the promoter sequence can still be considered “operably linked” to the coding sequence.

By “nucleic acid construct” it is meant a nucleic acid sequence that has been constructed to comprise one or more functional units not found together in nature. Examples include circular, linear, double-stranded, extrachromosomal DNA molecules (plasmids), cosmids (plasmids containing COS sequences from lambda phage), viral genomes comprising non-native nucleic acid sequences, and the like.

A “vector” is capable of transferring gene sequences to a host cell. Typically, “vector construct,” “expression vector,” and “gene transfer vector,” mean any nucleic acid construct capable of directing the expression of a gene of interest and which can transfer gene sequences to host cells, which can be accomplished by genomic integration of all or a portion of the vector, or transient or inheritable maintenance of the vector as an extrachromosomal element. Thus, the term includes cloning, and expression vehicles, as well as integrating vectors.

An “expression cassette” comprises any nucleic acid construct capable of directing the expression of a gene/coding sequence of interest, which is operably linked to a promoter of the expression cassette. Such cassettes can be constructed into a “vector,” “vector construct,” “expression vector,” or “gene transfer vector,” in order to transfer the expression cassette into a host cell. Thus, the term includes cloning and expression vehicles, as well as viral vectors.

A first polynucleotide is “derived from” or “corresponds to” a second polynucleotide if it has the same or substantially the same nucleotide sequence as a region of the second polynucleotide, its cDNA, complements thereof, or if it displays sequence identity as described above.

A first polypeptide is “derived from” or “corresponds to” a second polypeptide if it is (i) encoded by a first polynucleotide derived from a second polynucleotide, or (ii) displays sequence identity to the second polypeptides as described above.

The term “stable, folded protein insertion” refers to a folded region of polypeptide that is inserted between two neighboring domains (e.g., the TM5 and TM6 domains of a GPCR), such that the domains are spaced relative to each other at a distance that allows them to interact as in the wild-type protein. The term “stable, folded protein insertion” excludes an amino acid sequence of a fluorescent protein (e.g., GFP, CFP or YFP), and excludes amino acid sequences that are at least 90% identical to the entire IC3 loop of a GPCR. In general, the IC3 loops of wild type GPCRs do not contain stable, folded protein domains.

The term “active form” or “native state” of a protein is a protein that is folded in a way so as to be active. A GPCR is in its active form if it can bind ligand, alter conformation in response to ligand binding, and/or transduce a signal which may or may not be induced by ligand binding. An active or native protein is not denatured.

The term “stable domain” is a polypeptide domain that, when folded in its active form, is stable, i.e., does not readily become inactive or denatured.

The term “folds autonomously” indicates a protein that folds into its active form in a cell, without biochemical denaturation and renaturation of the protein, and without chaperones.

The term “naturally-occurring” refers to an object that is found in nature.

The term “non-naturally-occurring” refers to an object that is not found in nature.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As noted above, a fusion protein is provided. In certain embodiments, the fusion protein comprises: a) a first portion of a G-protein coupled receptor (GPCR), where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertion c) a second portion of the GPCR, where the second portion comprises the TM6 and TM7 regions of the GPCR. In particular embodiments, the stable, folded protein insertion spaces the ends of the TM5 region and the TM6 region of the GPCR at a distance in the range of 7 Å to 15 Å. The stable, folded protein insertion may also provide polar surface area for crystal lattice contacts.

In the following description, the fusion protein is described first, followed by a discussion of the crystallization method in which the fusion protein may be employed.

Fusion Proteins

As noted above, a subject fusion proteins comprises: a) a first portion of a G-protein coupled receptor (GPCR), where the first portion comprises the TM1, TM2, TM3, TM4 and TM5 regions of the GPCR; b) a stable, folded protein insertion c) a second portion of the GPCR, where the second portion comprises the TM6 and TM7 regions of the GPCR. In particular embodiments, the stable, folded protein insertion spaces the ends of the TM5 region and the TM6 region of the GPCR at a distance in the range of 7 Å to 15 Å. The stable, folded protein insertion may also provide polar surface for crystal lattice contacts.

In very general terms, such a protein may be made by substituting the IC3 region of the GPCR with a stable, folded protein that holds the two remaining portions of the GPCR (i.e. the portion that lies N-terminal to the IC3 region and the portion that lies C-terminal to the IC3 region) together at a distance that is compatible with a functional GPCR in terms of pharmacologic and dynamic properties.

GPCRs

Any known GPCR is suitable for use in the subject methods, as long as it has TM5 and TM6 regions that are identifiable in the sequence of the GPCR. A disclosure of the sequences and phylogenetic relationships between 277 GPCRs is provided in Joost et al. (Genome Biol. 2002 3:RESEARCH0063, the entire contents of which is incorporated by reference) and, as such, at least 277 GPCRs are suitable for the subject methods. A more recent disclosure of the sequences and phylogenetic relationships between 367 human and 392 mouse GPCRs is provided in Vassilatis et al. (Proc Natl Acad Sci 2003 100:4903-8 and www.primalinc.com, each of which is hereby incorporated by reference in its entirely) and, as such, at least 367 human and at least 392 mouse GPCRs are suitable for the subject methods. GPCR families are also described in Fredriksson et al (Mol. Pharmacol. 2003 63, 1256-72).

The methods may be used, by way of exemplification, for purinergic receptors, vitamin receptors, lipid receptors, peptide hormone receptors, non-hormone peptide receptors, non-peptide hormone receptors, polypeptide receptors, protease receptors, receptors for sensory signal mediator, and biogenic amine receptors not including β2-adrenergic receptor. In certain embodiments, said biogenic amine receptor does not include an adrenoreceptor. α-type adrenoreceptors (e.g. α_(1A), α_(1B) or α_(1C) adrenoreceptors), and β-type adrenoreceptors (e.g. β₁, β₂, or β₃ adrenoreceptors) are discussed in Singh et al., J. Cell Phys. 189:257-265, 2001.

It is recognized that both native (naturally occurring) and altered native (non-naturally occurring) GPCRs may be used in the subject methods. In certain embodiments, therefore, an altered native GPCR (e.g. a native GPCR that is altered by an amino acid substitution, deletion and/or insertion) such that it binds the same ligand as a corresponding native GPCR, and/or couples to a G-protein as a result of the binding. In certain cases, a GPCR employed herein may be at least 80% identical to, e.g., at least 90% identical, at least 85% identical, at least 90% identical, at least 95% identical, or at least 98% identical, to a naturally occurring GPCR.

As such, the following GPCRs (native or altered) find particular use as parental GPCRs in the subject methods: cholinergic receptor, muscarinic 3; melanin-concentrating hormone receptor 2; cholinergic receptor, muscarinic 4; niacin receptor; histamine 4 receptor; ghrelin receptor; CXCR3 chemokine receptor; motilin receptor; 5-hydroxytryptamine (serotonin) receptor 2A; 5-hydroxytryptamine (serotonin) receptor 2B; 5-hydroxytryptamine (serotonin) receptor 2C; dopamine receptor D3; dopamine receptor D4; dopamine receptor D1; histamine receptor H2; histamine receptor H3; galanin receptor 1; neuropeptide Y receptor Y1; angiotensin II receptor 1; neurotensin receptor 1; melanocortin 4 receptor; glucagon-like peptide 1 receptor; adenosine A1 receptor; cannabinoid receptor 1; and melanin-concentrating hormone receptor 1.

In particular embodiments, the GPCR may belong to one of the following GPCR families: amine, peptide, glycoprotein hormone, opsin, olfactory, prostanoid, nucleotide-like, cannabinoid, platelet activating factor, gonadotropin-releasing hormone, thyrotropin-releasing hormone or melatonin families, as defined by Lapinsh et al (Classification of G-protein coupled receptors by alignment-independent extraction of principle chemical properties of primary amino acid sequences. Prot. Sci. 2002 11:795-805) or family B (which includes the PTH and glucagon receptors) or family C (which includes the GABA and glutamate receptors).

In the subject methods, the region between the TM5 and TM6 regions of a GPCR (i.e., the IC3 region) is usually identified, and replaced with a stable, folded protein insertion to form a fusion protein. The stable, folded protein insertion spaces the TM5 and TM6 regions relative to one another. A schematic representation of the prototypical structure of a GPCR is provided in FIG. 1, where these regions, in the context of the entire structure of a GPCR, may be seen. A schematic representation of a subject fusion protein is shown in FIG. 2. In one embodiment, the IC3 loop of the GPCR is replaced with a stable, folded protein insertion.

The IC3 region of a GPCR lies in between transmembrane regions TM5 and TM6 and, may be about 12 amino acids (CXCR3 and GPR40) to about 235 amino acids (cholinergic receptor, muscarinic 3) in length, for example. The TM5, IC3, and TM6 regions are readily discernable by one of skill in the art using, for example, a program for identifying transmembrane regions; once transmembrane regions TM5 and TM6 regions are identified, the IC3 region will be apparent. The TM5, IC3, and TM6 regions may also be identified using such methods as pairwise or multiple sequence alignment (e.g. using the GAP or BESTFIT of the University of Wisconsin's GCG program, or CLUSTAL alignment programs, Higgins et al., Gene. 1988 73:237-44), using a target GPCR and, for example, GPCRs of known structure.

Suitable programs for identifying transmembrane regions include those described by Moller et al., (Bioinformatics, 17:646-653, 2001). A particularly suitable program is called “TMHMM” Krogh et al., (Journal of Molecular Biology, 305:567-580, 2001). To use these programs via a user interface, a sequence corresponding to a GPCR or a fragment thereof is entered into the user interface and the program run. Such programs are currently available over the world wide web, for example at the website of the Center for Biological Sequence Analysis at cbs.dtu.dk/services/. The output of these programs may be variable in terms its format, however they usually indicate transmembrane regions of a GPCR using amino acid coordinates of a GPCR.

When TM regions of a GPCR polypeptide are determined using TMHMM, the prototypical GPCR profile is usually obtained: an N-terminus that is extracellular, followed by a segment comprising seven TM regions, and further followed by a C-terminus that is intracellular. TM numbering for this prototypical GPCR profile begins with the most N-terminally disposed TM region (TM1) and concludes with the most C-terminally disposed TM region (TM7).

Accordingly, in certain embodiments, the amino acid coordinates of the TM5, IC-3, and TM6 regions of a GPCR are identified by a suitable method such as TMHMM.

In certain cases, once the TM5-IC3-TM6 segment is identified for a GPCR, a suitable region of amino acids is chosen for substitution with the amino acid sequence of the a stable, folded protein insertion. In certain embodiments, the substituted region may be identified using conserved or semi-conserved amino acids in the TM5 and TM6 transmembrane regions. In certain embodiments, the N-terminus of the a stable, folded protein insertion is linked to the amino acid that is 15 to 25 (e.g., 15, 16, 17, 18, 19, 20, 21, 22, 23, 24 or 25; e.g., 18-20) residues C-terminal to a conserved proline in the TM5 of the GPCR, although linkages outside of this region are envisioned. In certain embodiments, the C-terminus of the stable, folded protein insertion may be linked to the amino acid that is 20-30 (e.g., 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 or 30; e.g., 23-27) residues N-terminal a conserved proline in the TM6 region of the GPCR, although linkages outside of this region are envisioned.

For GPCRs that contain no conserved proline residues in TM5 and TM6, positions for inserting an a stable, folded protein insertion can be determined based on two considerations: a) alignment of the sequence of the GPCR with receptor members of the same subfamily (which contained conserved proline residues in TM5 or TM6; b) by identifying the juxtaposition to the TM5/TM6 regions by hydrophobicity analysis.

In addition to substituting IC3 region of a GPCR with a stable, folded protein insertion, as described above, in certain cases, the C-terminal region of the GPCR (which is C-terminal to the cysteine palmitoylation site that is approximately 10 to 25 amino acid residues downstream of a conserved NPXXY motif), may be deleted. In certain cases, the 20-30 amino acids immediately C-terminal to the cysteine palmitoylation site are not deleted.

Stable, Folded Protein Insertions

In certain embodiments, a stable, folded protein insertion of a subject fusion protein may be a soluble, stable protein (e.g., a protein displaying resistance to thermal and chemical denaturation) that folds autonomously of the GPCR portion of the fusion protein, in a cell. In certain cases, the stable, folded protein insertion may have no cysteine residues (or may be engineered to have no cysteine residues) in order to avoid potential disulphide bonds between the stable, folded protein insertion and a GPCR portion of the fusion protein, or internal disulphide bonds. Stable, folded protein insertions are conformationally restrained, and are resistant to protease cleavage.

In certain cases, stable, folded protein insertions may contain most or all of the amino acid sequence of a polypeptide that is readily crystallized. Such proteins may be characterized by a large number of deposits in the protein data bank (www.rcsb.org) in a variety of space groups and crystal packing arrangements. While examples that employ lysozyme as stable, folded protein insertion are discussed below, the general principles may be used to employ any of a number of polypeptides that have the characteristics discussed above. Suitable stable, folded protein insertion candidates include those containing the amino acid sequence of proteins that are readily crystallized including, but not limited to: lysozyme, glucose isomerase, xylanase, trypsin inhibitor, crambin, ribonuclease. Other suitable polypeptides may be found at the BMCD database (Gilliland et al 1994. The Biological Macromolecule Crystallization Database, Version 3.0: New Features, Data, and the NASA Archive for Protein Crystal Growth Data. Acta Crystallogr. D50 408-413), as published to the world wide web.

In certain embodiments, the stable, folded protein insertion used may be at least 80% identical (e.g., at least 85% identical, at least 90% identical, at least 95% identical or at least 98% identical to a wild type protein. Many suitable wild type proteins, including non-naturally occurring variants thereof, are readily crystallizable.

As noted above, one such stable, folded protein insertion that may be employed in a subject fusion protein is lysozyme. Lysozyme is a highly crystallizable protein (see, e.g., Strynadka et al Lysozyme: a model enzyme in protein crystallography EXS 1996 75: 185-222) and at present over 200 atomic coordinates for various lysozymes, including many wild-type lysozymes and variants thereof, including lysozymes from phage T4, human, swan, rainbow trout, guinea fowl, soft-shelled turtle, tapes japonica, nurse shark, mouse sperm, dog and phage P1, as well as man-made variants thereof, have been deposited in NCBI's structure database. A subject fusion protein may contain any of a wide variety of lysozyme sequences.

The length of the stable, folded protein insertion may be between 80-500 amino acids, e.g., 100-200 amino acids in length, although stable, folded protein insertions having lengths outside of this range are also envisioned.

As noted above, the stable, folded protein insertion is not fluorescent or light-emitting. As such, the stable, folded protein insertion is not CFP, GFP, YFP, luciferase, or other light emitting, fluorescent variants thereof. In certain cases, a stable, folded protein insertion region does not contain a flexible polyglycine linker or other such conformationally unrestrained regions. In certain cases, the stable, folded protein insertion contains a sequence of amino acids from a protein that has a crystal structure that has been solved. In certain cases, the stable, folded protein insertion should not have highly flexible loop region characterized by high cyrstallographic temperature factors (i.e., high B-factors).

In general terms, once a suitable polypeptide is identified, a stable, folded protein insertion may be designed by deleting amino acid residues from the N-terminus, the C-terminus or both termini of the polypeptide such that the closest alpha carbon atoms in the backbone at the termini of the polypeptide are spaced by a distance of in the range of 6 Å to 16 Å, e.g., 7 Å to 15 Å, 7 Å to 10 Å, 12 Å to 15 Å, 10 Å to 13 Å, or about 11 Å (i.e. 10 Å to 12 Å). The stable, folded protein insertion, disposed between the TM5 and TM6 regions of a GPCR, spaces those regions by that distance. The distance may be modified by adding or removing amino acids to or from the stable, folded protein insertion.

Amino acid sequence for exemplary lysozyme fusion proteins are set forth in FIG. 9, and the amino acid sequences of exemplary alternative insertions (which may be substituted into any of the sequences of FIG. 9 in place of the lysozyme sequence) are shown in FIG. 16. These sequences include the sequences of trypsin inhibitor, calbindin, barnase, xylanase and glucokinase although other sequences can be readily used.

Nucleic Acids

A nucleic acid comprising a nucleotide sequence encoding a subject fusion protein is also provided. A subject nucleic acid may be produced by any method. Since the genetic code and recombinant techniques for manipulating nucleic acid are known, the design and production of nucleic acids encoding a subject fusion protein is well within the skill of an artisan. In certain embodiments, standard recombinant DNA technology (Ausubel, et al, Short Protocols in Molecular Biology, 3rd ed., Wiley & Sons, 1995; Sambrook, et al., Molecular Cloning: A Laboratory Manual, Second Edition, (1989) Cold Spring Harbor, N.Y.) methods are used.

For example, site directed mutagenesis and subcloning may be used to introduce/delete/substitute nucleic acid residues in a polynucleotide encoding GPCR. In other embodiments, PCR may be used. Nucleic acids encoding a polypeptide of interest may also be made by chemical synthesis entirely from oligonucleotides (e.g., Cello et al., Science (2002) 297:1016-8).

In certain embodiments, the codons of the nucleic acids encoding polypeptides of interest are optimized for expression in cells of a particular species, particularly a mammalian, e.g., human, species. Vectors comprising a subject nucleic acid are also provided. A vector may contain a subject nucleic acid, operably linked to a promoter.

A host cell (e.g., a host bacterial, mammalian, insect, plant or yeast cell) comprising a subject nucleic acid is also provided as well a culture of subject cells. The culture of cells may contain growth medium, as well as a population of the cells. The cells may be employed to make the subject fusion protein in a method that includes culturing the cells to provide for production of the fusion protein. In many embodiments, the fusion protein is directed to the plasma membrane of the cell, and is folded into its active form by the cell.

The native form of a subject fusion protein may be isolated from a subject cell by conventional technology, e.g., by precipitation, centrifugation, affinity, filtration or any other method known in the art. For example, affinity chromatography (Tilbeurgh et al., (1984) FEBS Lett. 16:215); ion-exchange chromatographic methods (Goyal et al., (1991) Biores. Technol. 36:37; Fliess et al., (1983) Eur. J. Appl. Microbiol. Biotechnol. 17:314; Bhikhabhai et al., (1984) J. Appl. Biochem. 6:336; and Ellouz et al., (1987) Chromatography 396:307), including ion-exchange using materials with high resolution power (Medve et al., (1998) J. Chromatography A 808:153; hydrophobic interaction chromatography (Tomaz and Queiroz, (1999) J. Chromatography A 865:123; two-phase partitioning (Brumbauer, et al., (1999) Bioseparation 7:287); ethanol precipitation; reverse phase HPLC; chromatography on silica or on a cation-exchange resin such as DEAE; chromatofocusing; SDS-PAGE; ammonium sulfate precipitation; or size exclusion chromatography using, e.g., Sephadex G-75, may be employed.

In particular embodiments, the GPCR, e.g., the N- or C-terminus of the GPCR or an external loop of the GPCR, may be tagged with an affinity moiety, e.g., a his tag, GST, MBP, flag tag, or other antibody binding site, in order to facilitate purification of the GPCR fusion protein by affinity methods.

Before crystallization, a subject fusion protein may be assayed to determine if the fusion protein is active, e.g., can bind ligand and change in conformation upon ligand binding, and if the fusion protein is resistant to protease cleavage. Such assays are well known in the art.

In certain cases the subject fusion protein may be combined with a ligand for the GPCR of the fusion protein prior to crystallization.

Crystallization Methods

A subject fusion protein may be crystallized using any of a variety of crystallization methods, many of which are reviewed in Caffrey Membrane protein crystallization. J Struct. Biol. 2003 142:108-32. In general terms, the methods are lipid-based methods that include adding lipid to the fusion protein prior to crystallization. Such methods have previously been used to crystallize other membrane proteins. Many of these methods, including the lipidic cubic phase crystallization method and the bicelle crystallization method, exploit the spontaneous self-assembling properties of lipids and detergent as vesicles (vesicle-fusion method), discoidal micelles (bicelle method), and liquid crystals or mesophases (in meso or cubic-phase method). Lipidic cubic phases crystallization methods are described in, for example: Landau et al, Lipidic cubic phases: a novel concept for the crystallization of membrane proteins. Proc. Natl. Acad. Sci. 1996 93: 14532-5; Gouaux, It's not just a phase: crystallization and X-ray structure determination of bacteriorhodopsin in lipidic cubic phases. Structure. 1998 6:5-10; Rummel et al, Lipidic Cubic Phases: New Matrices for the Three-Dimensional Crystallization of Membrane Proteins. J. Struct. Biol. 1998 121:82-91; and Nollert et al Lipidic cubic phases as matrices for membrane protein crystallization Methods. 2004 34:348-53, which publications are incorporated by reference for disclosure of those methods. Bicelle crystallization methods are described in, for example: Faham et al Crystallization of bacteriorhodopsin from bicelle formulations at room temperature. Protein Sci. 2005 14:836-40. 2005 and Faham et al, Bicelle crystallization: a new method for crystallizing membrane proteins yields a monomeric bacteriorhodopsin structure. J Mol Biol. 2002 Feb. 8; 316(1):1-6, which publications are incorporated by reference for disclosure of those methods.

Also provided is a method of determining a crystal structure. This method may comprise receiving an above described fusion protein, crystallizing the fusion protein to produce a crystal; and obtaining atomic coordinates of the fusion protein from the crystal. The fusion protein may be received from a remote location (e.g., a different laboratory in the same building or campus, or from a different campus or city), and, in certain embodiments, the method may also comprise transmitting the atomic coordinates, e.g., by mail, e-mail or using the interne, to the remote location or to a third party.

In other embodiments, the method may comprise forwarding a fusion protein to a remote location where the protein may be crystallized and analyzed, and receiving the atomic coordinates of the fusion protein.

In order to further illustrate the present invention, the following specific examples are given with the understanding that they are being offered to illustrate the present invention and should not be construed in any way as limiting its scope.

METHODS

Molecular Biology for Generation of Mammalian and Sf9 Expression Constructs.

The insect cell expression plasmid that was used as a template for modification of the human β₂AR gene has been described previously (X. Yao et al., Nat Chem Biol 2, 417 (2006)): the wild-type coding sequence of the human β₂AR (starting at Gly2) was cloned into the pFastbac1 Sf-9 expression vector (Invitrogen) with the HA signal sequence followed by the Flag epitope tag at the amino terminus and the third glycosylation site mutated as N187E. Using this template, a TAA stop codon was placed between Gly365 and Tyr366, terminating translation without the 48 C-terminal residues of the wild-type β₂AR (“βAR365”). A synthetic DNA cassette encoding the T4 Lysozyme (WT*-C54T, C97A) protein was made by overlapping extension PCR of 50-base oligonucleotides. This cassette was amplified and inserted into the β₂AR365 construct between Ile233^(5.72) and Arg260^(6.22) (“E1” in FIG. 3A), using the Quickchange Multi protocol (Stratagene). The corresponding mammalian cell expression plasmid was made by amplifying the entire fusion gene and cloning it into pcDNA3 (Invitrogen). Further deletions in the Sf9 and mammalian cell constructs were made using appropriate synthetic oligonucleotides in the Quickchange Multi protocol (Stratagene). All constructs were confirmed by sequencing.

HEK293 Cell Staining and Immunofluorescence Staining.

HEK293 cells were cultured on plastic dishes at 37° C. with 5% CO₂ in Dulbecco's modified Eagle's medium (Cellgro) with 5% fetal bovine serum. For an individual expression experiment, cells at confluency were split, and approximately 100,000 cells were used to seed glass cover slips in the same medium. After 2 d, cells were transfected with the addition of 1 μg of a given pcDNA3-receptor plasmid and 3 μl of Fugene 6 reagent (Roche). 48 h after transfection, cells were washed with PBS, fixed with 4% paraformaldehyde, blocked with PBS+2% goat serum, permeabilized with PBS+2% goat serum +0.5% Nonidet P-40 (Sigma), stained with Alexa488-conjugated M1 anti-FLAG antibody (for receptor) plus DAPI (nuclear) in blocking buffer, and washed with blocking buffer. Cover slips were mounted on microscope slides with Vectashield (Vector Labs) and dried overnight. Staining was visualized with an Axioplan 2 fluorescence imaging system, using a 63× objective and either green (Alexa488/FITC) or blue (DAPI/Hoechst) filter sets. A plasmid pcDNA3-β₁AR, expressing an N-terminal FLAG-tagged β₁ adrenergic receptor, was used as a positive control for cell-surface staining. Empty pcDNA3 was used as a negative control to assess background staining.

Expression and Purification of β₂AR-T4L from Baculovirus-Infected Sf9 Cells.

Recombinant baculovirus was made from pFastbac1-β₂AR-T4L using the Bac-to-Bac system (Invitrogen), as described previously (X. Yao et al., Nat Chem Biol 2, 417 (2006)). The β₂AR-T4L protein was expressed in Sf9 insect cells infected with this baculovirus, and solubilized according to previously described methods (B. K. Kobilka, Anal Biochem 231, 269 (1995)). Dodecylmaltoside-solubilized receptor with the N-terminal FLAG epitope (DYKDDDA) was purified by M1 antibody affinity chromatography (Sigma), treated with TCEP/iodoacetamide, and further purified by alprenolol-Sepharose chromatography (2) to isolate only functional GPCR. Eluted alprenolol-bound receptor was re-bound to M1 FLAG resin, and ligand exchange with 30 μM carazolol was performed on the column. β₂AR-T4L was eluted from this final column with 0.2 mg/ml FLAG peptide in HLS buffer (0.1% dodecylmaltoside, 20 mM Hepes, 100 mM NaCl, pH 7.5) plus 30 μM carazolol and 5 mM EDTA. N-linked glycolsylations were removed by treatment with PNGaseF (NEB). Protein was concentrated from ˜5 mg/ml to 50 mg/ml with a 100 kDa molecular weight cut-off Vivaspin concentrator (Vivascience), and dialyzed against HLS buffer plus 10 μM carazolol.

Binding Measurements on Wild-Type β₂AR and β₂AR-T4L from Membranes.

Membrane preparation from baculovirus-infected Sf9 cells was performed as described previously (G. Swaminath, J. Steenhuis, B. Kobilka, T. W. Lee, Mol Pharmacol 61, 65 (2002)). For each binding reaction, membranes containing 0.7 μg total membrane protein were used. Saturation binding of [³H]-dihydroalprenolol (DHA) was measured by incubating membranes resuspended in 500 μl binding buffer (75 mM Tris, 12.5 mM MgCl₂, 1 mM EDTA, pH 7.4, supplemented with 0.4 mg/ml BSA) with 12 different concentrations of [³H]DHA (Perkin Elmer) between 20 pM and 10 nM. After 1 h incubation with shaking at 230 rpm, membranes were filtered from the binding reactions with a Brandel harvester, washed with binding buffer, and measured for bound [³H]DHA with a Beckman LS6000 scintillation counter. Non-specific binding was assessed by performing identical reactions in the presence of 1 μM alprenolol. For competition binding, membranes resuspended in 500 μA binding buffer were incubated with 0.5 nM [³H]DHA plus increasing concentrations of the competing ligand (all compounds were purchased from Sigma). For (−)-isoproterenol and (−)-epinephrine, concentrations were 100 pM-1 mM, each increasing by a factor of 10. For salbutamol, concentrations were 1 nM-10 mM. For ICI-118,551 and formoterol, concentrations were 1 pM-10 μM. Non-specific binding was measured by using 1 μM unlabeled alprenolol as competing ligand. Each data point in the curves in FIGS. 4A and 10 represents the mean of three separate experiments, each done in triplicate. Binding data were analyzed by nonlinear regression analysis using Graphpad Prism. The values for K_(d) of [³H]DHA and K_(i) of other ligands are shown in Table S1.

Bimane Fluorescence Experiments on Purified, Detergent-Solubilized Receptors

β₂AR-T4L and β₂AR365 were purified as described above, with two differences. First, prior to iodoacetamide treatment, FLAG-pure receptor at 2.5 μM (measured by soluble [³H]DHA binding) was incubated with 5 μM monobromobimane for 1 h at 4° C. Second, after binding the bimane-labeled alprenolol-Sepharose-purified receptor to M1 antibody resin, the column was washed extensively with ligand-free buffer before elution. Based on previous precedent, this protocol is expected to target primarily Cys265^(6.27) for fluorophore derivitization. Fluorescence spectroscopy was performed on a Spex FluoroMax-3 spectrofluorometer (Jobin Yvon Inc.) with photon-counting mode, using an excitation and emission bandpass of 5 nm. All experiments were done at 25° C. For emission scans, we set excitation at 350 nm and measured emission from 417 to 530 nm with an integration time of 1.0 s nm⁻¹. To determine the effect of ligands, spectra were measured after 15 min incubation with different compounds (at saturating concentrations−[(−)-isoproterenol]=100 μM, [ICI-118,551]=10 μM, [salbutamol]=500 μM). Fluorescence intensity was corrected for background fluorescence from buffer and ligands in all experiments. The curves shown in FIG. 4B are each the average of triplicate experiments performed in parallel. λ_(max) values and intensity changes for β₂AR-T4L and β₂AR365, each incubated with different ligands, are tabulated in Table S2.

Comparing the Proteolytic Stability of Unliganded β₂AR and β₂AR-T4L.

The limited trypsin proteolysis protocol was adapted from Jiang et al. (Z. G. Jiang, M. Carraway, C. J. McKnight, Biochemistry 44, 1163 (2005)). Carazolol-bound β₂AR-T4L or wild-type β₂AR (each at 30 mg/ml) were diluted 10-fold into HLS buffer (see above) and TPCK-trypsin was added at a 1:1000 ratio (wt:wt). The digests were incubated at room temperature. At various time points, aliquots were removed and flash frozen on dry ice/ethanol. After the last aliquot was removed, all samples were thawed, and an equal volume of 10% SDS/PAGE loading buffer was added to each. Samples were then analyzed by electrophoresis on 12% polyacrilamide gels, followed by staining with Coomassie blue. See FIG. 11.

Comparing the Stability of Unliganded β₂AR and β₂AR-T4L

Unliganded β₂AR365 and β₂AR-T4L were each purified as described above for the bimane experiments. 200 μl 0.02 mg/ml receptor in HLS buffer was incubated at 37° C. on a heating block. At the time points indicated in FIG. 12, samples were briefly spun and gently vortexed and 16.5 μl was removed and diluted 18.2-fold in HLS (300 μl total). Then 4×5 μl was removed for determination of total binding and 2×5 μl was removed for nonspecific binding. To measure soluble binding, 5 μl diluted receptor was added to 105 μl HLS (400-fold final dilution of receptor) containing 10 nM [³H]DHA±10 μM cold alprenolol. Reactions were incubated 30 min at RT, then on ice until processing. 100 μl of each reaction was applied to a 1 ml G50 column to separate protein from residual unbound [³H]DHA, and receptor was eluted using 1.1 ml ice-cold HLS. Bound [³H]DHA was quantified on a Beckman LS6000 scintillation counter.

Carazolol Dissociation from the “Wild-Type” Receptor β₂AR365

β₂AR365 was purified with carazolol bound, according to the protocol described above for β₂AR-T4L. Carazolol-bound receptor (at approximately 50 μM concentration) was dialyzed in the dark against 1 L dialysis buffer (20 mM HEPES pH7.5, 100 mM NaCl, 0.1% dodecylmaltoside, 300 micromolar alprenolol) at room temperature with stirring. At indicated time points, two samples were removed from the parafilm-sealed open-ended dialysis chamber, diluted into fresh dialysis buffer, and carazolol emission spectra were obtained on a Spex FluoroMax spectrofluorometer (using excitation at 330 nm and emission from 335 to 400 nm). As internal standards for every time point, samples were removed for determination of protein concentration using the Bio-Rad Protein DC kit. See FIG. 14.

CAM and UCM Mutants

The CAMs (constitutively active mutants) described in the literature that are the basis for FIG. 8A and the associated discussion are: L124A, C116F, D130A, L272C, and C285T. The UCMs (uncoupling mutations) from the literature that were used are: D79N, F139A, T1641, N318K, N322A, P323A, Y326A, L339A, and L340A.

TABLE S1 Binding affinities of different ligands for the wild-type β₂AR and the fusion protein β₂AR-T4L. The saturation and competition binding curves shown in FIG. 4 were fit to theoretical saturation and one-site competition binding models, using the program Graphpad Prism. K_(i) values were calculated using the Cheng-Prusoff equation: K_(i) = IC₅₀/(1 + [ligand]/K_(d)) Saturation Binding [³H]DHA K_(d) ± SE (nM) Bmax (pmol/mg) β₂AR 0.161 ± 0.012 30.0 ± 0.5 β₂AR-T4L 0.180 ± 0.016 21.6 ± 0.5 Competition Binding K_(i) [S.E. interval] for K_(i) [S.E. interval] for Ligand β₂AR (nM) β₂AR-T4L (nM) (−)-isoproteronol  50.6 [48.9-52.3]  15.7 [15.2-16.2] (−)-epinephrine   175 [163-188]  56.0 [52.8-59.4] salbutamol   728 [708-750]   307 [291-323] ICI-118,551 0.617 [0.570-0.668] 0.626 [0.591-0.662] formoterol  3.60 [3.39-3.83]  1.68 [1.55-1.81]

TABLE S2 Bimane fluorescence responses for unliganded β₂AR365 and β₂AR-T4L, incubated for 15 min with different ligands. Top panel shows the λ_(max) for fluorescence emission spectra (excitation at 350 nm and emission from 417 to 530 nm) collected after 15 min incubation with ligand. Each value is mean ± standard deviation for triplicate experiments performed in parallel. Bottom panel shows the change in fluorescence intensity after incubation with ligand, represented as the ratio of Intensity at λmax of the ligand to Intensity at λ_(max) of the control no ligand (“none”) response. λmax ± SD λmax ± SD for for β₂AR- Ligand β₂AR365 (nm) T4L (nm) none 448 ± 2 447 ± 2 (−)-isoproteronol 453 ± 2 455 ± 2 ICI-118,551 447 ± 1 446 ± 1 salbutamol 449 ± 1 449 ± 1 Intensity at λmax_(Ligand)/Intensity at λmax_(none) Ligand β₂AR365 β₂AR-T4L (−)-isoproteronol 0.758 ± 0.007 0.824 ± 0.006 ICI-118,551 1.013 ± 0.008 1.028 ± 0.008 salbutamol 0.950 ± 0.013 0.928 ± 0.009

TABLE S3 Buried surface area contributions at the β₂AR-T4L/carazolol interface. Solvent accessible surface area calculations were done with the CNS software package, using a probe radius of 1.4 Å. Buried surface area contributions of individual residues were determined by calculating solvent-accessible surface area per residue for the full β₂AR-T4L/ carazolol model, and subtracting these numbers from the calculated values for the receptor model without carazolol. Surface area β₂AR residue buried (Å²) Trp109^(3.28) 21.4 Thr110^(3.29) 5.7 Asp113^(3.32) 19.3 Val114^(3.33) 25.5 Val117^(3.36) 8.5 Thr118^(3.37) 1.9 Phe193^(5.32) 51.2 Thr195^(5.34) 7.4 Tyr199^(5.38) 7.6 Ala200^(5.39) 10.0 Ser203^(5.42) 9.0 Ser204^(5.43) 4.6 Ser207^(5.46) 6.3 Trp286^(6.48) 3.1 Phe289^(6.51) 20.0 Phe290^(6.52) 19.0 Phe293^(6.55) 18.7 Tyr308^(7.35) 14.4 Asn312^(7.39) 22.5 Tyr316^(7.43) 6.5 Lipidic Cubic Phase Crystallization

For lipidic cubic phase (LCP) crystallization trials, robotic trials were performed using an in meso crystallization robot. 96-well glass sandwich plates (S1, S2) were filled with 25 or 50 nL protein-laden LCP drops overlaid by 0.8 μL of precipitant solution in each well and sealed with a glass coverslip. All operations starting from mixing lipid and protein were performed at room temperature (˜21-23° C.). Crystals were obtained in 30-35% (v/v) PEG 400, 0.1-0.2 M sodium sulfate, 0.1 M Bis-tris propane pH 6.5-7.0 and 5-7% (v/v) 1,4-butanediol using 8-10% (w/w) cholesterol in monoolein as the host lipid. PEG 400 and sulfate ion were used for crystallization, and the addition of cholesterol and 1,4-butanediol improved crystals size and shape enabling high-resolution diffraction. Additions of phospholipids (dioleoylphosphatidylcholine, dioleoylphosphatidylethanolamine, asolectin) alone and in combinations with cholesterol to the main host LCP lipid monoolein were tried, however, none of them improved crystal quality.

Crystal Harvesting

The average size of the harvested crystals was 30×15×5 μm (largest crystal was 40×20×7 μm). Crystals were harvested directly from the glass sandwich plates, even though these plates have been specifically designed for screening and optimization (S1, S2). Crystals were scooped directly from the LCP using 30 or 50 μm aperture MiTeGen MicroMounts and plunged into liquid nitrogen. Care was taken to drag as little as possible lipid around the crystal to decrease unwanted background scattering. Attempts to dissolve the lipids, either by increasing concentration of PEG 400 or using a mineral oil, typically resulted in a decrease in diffraction power of the crystals.

Data Collection

X-ray data were collected on the 231D-B beamline (GM/CA CAT) at the Advanced Photon Source, Argonne, IL using a 10 μm minibeam (wavelength 1.0332 Å) and a MarMosaic 300 CCD detector. Several complete datasets were collected from single crystals at resolution between 2.8 and 3.5 Å using 5× attenuated beam, 5 s exposure and 1° oscillation per frame. However, some crystals diffracted to a maximum of 2.2 Å resolution upon 5 s exposure with 1× attenuated beam. Therefore, we collected 10-20° wedges of high-resolution data from more than 40 crystals (some of the crystals were large enough to allow 2-3 translations) and combined 31 of the best datasets together from 27 independent crystals, scaling them against the lower resolution full dataset to obtain complete 2.4 Å data.

One of the challenges during data collection was visualization of colorless microcrystals within an opaque frozen lipid phase and aligning them with the 10 μm minibeam. Without being able to visualize the crystals adequately through the inline optics at the beamline, we resorted to alignment by diffraction. After numerous trial-and-error attempts, an optimized crystal search algorithm was designed to locate the crystals without the minibeam. First, the area of the loop containing lipid was scanned in the vertical direction with a highly attenuated and slitted 100×25 μm beam. When diffraction was found, the crystal location was further confined by two additional exposures to an area of ˜50×25 μm. This area was further coarse-scanned with the collimated and 10× attenuated minibeam using 15 μm steps, following by fine-tuning the position using 5 and 2 μm steps. After locating the crystal in one orientation the loop was rotated 90° and the procedure was repeated. Typically during alignment the crystal was exposed ˜10 times using 10× attenuated beam and 2 s exposures. Work is in progress to develop a fully automated scanning procedure to align invisible microcrystals with the minibeam in place.

Data Processing

A 90% complete, 2-fold redundant monoclinic dataset was processed from one crystal diffracting to 2.8 Å resolution. Initial indexing of lattice parameters in spacegroup C2 and crystal orientation were performed using HKL2000. The refined lattice parameters and space group were implemented in the data processing program XDS for spot integration which models error explicitly for radiation decay, absorption, and rotation. The 2.8 Å data was used as a scaling reference for incorporation of additional wedges of data collected at a much higher exposure. Each new dataset was indexed in XDS using the original unit cell parameters as constants which were then refined along with the crystal orientation, beam geometry, and mosaicity parameters. The refinement was generally stable, resulting in very similar unit cell constants which enabled subsequent scaling. All of the integrated wedges of data were then tested individually against the scaling reference set and included in the final scaled dataset if the merging statistics remained acceptable upon incorporation of the data. In total, 31 wedges of data from 27 crystals were combined with the scaling reference dataset, 22 of which diffracted to a resolution of 2.4 Å or better. Each of the higher resolution datasets were exposed to a much larger dose of radiation resulting in a rapid decay in intensity. Typically 10°-20° wedges were collected from each crystal or translation, 5°-7° of which had diffraction data to 2.4 Å. Based on the mean F/σ(F) of reflections near the three crystallographic axes, we estimate the effective resolution to be 2.4 Å along b* and c* and 2.7 Å along a*. The anisotropy results in the high merging R factors in the last few resolution shells despite the significant I/σ(I) values. The anisotropy is either an inherent property of the crystals or the result of a preferential orientation of the crystals within the mounting loop. Thus, the higher resolution shells were filled in anisotropically by incorporation of the additional data at high exposure levels, while the lower resolution shells have a very high redundancy and low anisotropy.

EXAMPLE 1 Summary of Results

In order to obtain high-resolution structural information on the β₂AR, most of the third intracellular loop (ICL3) was replaced by the protein T4 lysozyme (T4L). The C-terminal tail was also eliminated. The optimized β₂AR-T4L protein was crystallized in lipidic cubic phase, and the resulting 2.4 Å resolution crystal structure reveals the interface between the receptor and the ligand carazolol, a partial inverse agonist. Analysis of mutagenesis data in light of the structure clarifies the roles of different amino acids in inverse agonist binding, and implies that rearrangement of the binding pocket accompanies agonist binding. In addition, the structure reveals how mutations known to cause constitutive activity or uncoupling of agonist binding and G-protein activation are distributed between the ligand-binding pocket and the cytoplasmic surface of the protein, such that changes in side chains due to interaction with the ligand can be transmitted through the structure to the site of G protein interaction.

EXAMPLE 2 β₂AR-T4L A Crystallizable GPCR Fusion Protein

β₂AR crystallization was done by replacing the ICL3 of that protein with a well-structured, soluble domain that aids in the formation of lattice contacts. The initial criteria for choosing the inserted soluble protein were that the amino and carboxyl termini would approximate the predicted distance between the cytoplasmic ends of helix V and helix VI, and that the protein would crystallize under a variety of conditions. T4L is a small, stable protein that fulfills these criteria. The amino and carboxyl termini of wild-type T4L are 10.7 Å apart in PDB 2LZM, compared to a distance of 15.9 Å between the carbonyl carbon of residue 228^(5.63) and the amide nitrogen of residue 241^(6.24) in the high-resolution structure of rhodopsin (PDB 1U19).

DNA encoding the T4L protein (C54T, C97A) (M. Matsumura, W. J. Becktel, M. Levitt, B. W. Matthews, Proc Natl Acad Sci USA 86, 6562 (1989)) was initially cloned into the human β₂AR gene, guided by comparison of ICL3 length and sequence among class A GPCRs (F. Horn et al., Nucleic Acids Res 31, 294 (2003)): residues 234^(5.73)-259^(6.21) of the β₂AR were replaced by residues 2-164 of T4L (construct “E3” in FIG. 3A). In addition, the receptor was truncated at position 365, which aligns approximately with the position of the rhodopsin carboxyl terminus. Although these modifications resulted in a receptor that was expressed efficiently in Sf9 cells, further optimization was carried out to reduce the length of the junction between the receptor and the T4L termini. Several candidate constructs are illustrated in FIG. 3A, and selected immunofluorescence images of transfected, permeabilized HEK293 cells are shown in FIG. 3B. Relative to the initial construct, we could remove three residues from the cytoplasmic end of helix V, three residues from the C-terminal end of T4L, and three residues from the N terminus of helix VI, all without losing significant cell-surface expression. The final construct used for crystallization trials (“β₂AR T4L”) has residues 231^(5.70)-262^(6.24) of the β₂AR replaced by amino acids 2-161 of T4L (“1D” in FIG. 3A).

EXAMPLE 3 Functional Properties of β₂AR-T4L

Saturation binding of [³H]DHA to the β₂AR-T4L was measured, as well as competition binding of the inverse agonist ICI-118,551 and several agonists (FIGS. 4A and 10 and Table S1). The results show that β₂AR-T4L has wild-type affinity for the antagonist [³H]DHA and the inverse agonist ICI-118,551, whereas the affinity for both agonists (isoproterenol, epinephrine, formoterol) and a partial agonist (salbutamol) is two to three-fold higher relative to wild-type β₂AR. Higher agonist binding affinity is a property associated with constitutively active mutants (CAMs) of GPCRs. CAMs of the β₂AR also exhibit elevated basal, agonist-independent activation of Gs, and typically have lower expression levels and reduced stability. β₂AR-T4L exhibits binding properties of a CAM, but it expresses at levels exceeding 1 mg per liter of Sf9 cell culture, is more resistant to trypsin proteolysis than the wild-type β₂AR (FIG. 11), and retains binding activity in detergent at 37° C. as well as the wild-type receptor (FIG. 12).

β₂AR-T4L did not couple to G_(s), as expected due to the replacement of ICL3 by T4L. To assess whether the fused protein alters receptor function at the level of its ability to undergo conformational changes, we used a covalently attached fluorescent probe as a reporter for ligand-induced structural changes. Fluorophores attached at Cys265^(6.27), at the cytoplasmic end of helix VI, detect agonist-induced conformational changes that correlate with the efficacy of the agonist towards G protein activation. Detergent-solubilized β₂AR365 (wild-type receptor truncated at 365) and β₂AR-T4L were each labeled with monobromobimane, which has been used previously to monitor conformational changes of the β₂AR. Addition of the agonist isoproterenol to purified β₂AR365 induces a decrease in fluorescence intensity and a shift in λ_(max) for the attached bimane probe (FIG. 4B and Table S2). These changes in intensity and λ_(max) are consistent with an agonist-induced increase in polarity around bimane. A smaller change is observed with the partial agonist salbutamol, while the inverse agonist ICI-118,551 had little effect. For the β₂AR-T4L, there are subtle differences in the baseline spectrum of the bimane-labeled fusion protein, as might be expected if the environment around Cys265^(6.27) is altered by T4L. However, the full agonist isoproterenol induces a qualitatively similar decrease in intensity and rightward shift in λ_(max). Thus the presence of the fused T4L does not prevent agonist-induced conformational changes. The partial agonist salbutamol induced larger responses in β₂AR-T4L than were observed in wild-type β₂AR, and there was a small increase in fluorescence in response to the inverse agonist ICI-118,551. These are properties observed in CAMs and are consistent with the higher affinities for agonists and partial agonists exhibited by β₂AR-T4L. Therefore, we conclude that the T4L fusion induces a partial constitutively active phenotype in the β₂AR, likely caused by changes at the cytoplasmic ends of helices V and VI.

EXAMPLE 4 Comparison Between β₂AR-T4L and β₂AR-Fab Structures

The β₂AR-T4L fusion strategy is validated by comparison of its structure to the structure of wild-type β₂AR complexed with a Fab that recognizes a three dimensional epitope consisting of the amino and carboxyl-terminal ends of ICL3, determined at an anisotropic resolution of 3.4 Å/3.7 Å. FIG. 5A illustrates the similarity between the fusion and antibody complex approaches to β₂AR crystallization, in that both strategies rely on attachment (covalent or non-covalent, respectively) of a soluble protein partner between helices V and VI. A major difference between the two structures is that the extracellular loops and the carazolol ligand could not be modeled in the β₂AR-Fab complex, whereas these regions are resolved in the structure of β₂AR-T4L. Nonetheless, it is clear that the T4L insertion does not significantly alter the receptor. Superposition of the two structures (FIG. 13) illustrates that the transmembrane helices of the receptor components are very similar (RMSD=0.8 Å for 154 common modeled transmembrane Cα positions, versus 2.3 Å between β₂AR-T4L and the 154 equivalent residues in rhodopsin), especially when the modest resolution of the Fab complex is taken into account.

There is one significant difference between the Fab-complex and chimeric receptor structures that can be attributed to the presence of T4L. The cytoplasmic end of helix. VI is pulled outward as a result of the fusion to the carboxyl terminus of T4L, which alters the packing of Phe264^(6.26) at the end of helix VI (FIG. 5B). In the Fab-complex β₂AR, interactions between Phe264^(6.26) and residues in helix V, helix VI, and ICL2 may be important in maintaining the β₂AR in the basal state. The loss of these packing interactions in β₂AR-T4L could contribute to the higher agonist binding affinity characteristic of a CAM.

An unexpected difference between the structure of rhodopsin and the β₂AR-T4L involves the sequence E/DRY found at the cytoplasmic end of helix III in 71% of class A GPCRs. In rhodopsin, Glu134^(3.49) and Arg135^(3.50) form a network of hydrogen bond and ionic interactions with Glu247^(6.30) at the cytoplasmic end of helix VI. These interactions have been referred to as an “ionic lock” that stabilizes the inactive state of rhodopsin and other class A members. However, the arrangement of the homologous residues is significantly different in β₂AR-T4L: Arg131^(3.50) interacts primarily with Asp130^(3.49) and a sulfate ion rather than with Glu268^(6.30), and the distance between helix III and helix VI is greater than in rhodopsin (FIG. 5C). This difference might be explained by the interaction between Glu268^(6.30) and Arg8 of T4L; however, the arrangement of Asp130^(3.49) and Arg131^(3.50) and the distance between helix III and helix VI is very similar to that observed in the β₂AR-Fab structure. While the presence of an antibody or T4L at the ICL3 region could potentially affect the arrangement of these residues, the fact that similar ionic lock structures were obtained using two different approaches suggests that a broken ionic lock may be a genuine feature of the carazolol-bound state of the receptor.

EXAMPLE 5 Ligand Binding to the β₂AR

The β₂AR-T4L fusion protein was purified and crystallized in complex with the inverse agonist carazolol. Carazolol stabilizes the β₂AR against extremes of pH and temperature, perhaps related to its unusually high binding affinity (K_(d)<0.1 nM) and slow dissociation kinetics (t_(1/2)˜30 h) (FIG. 14). The interactions between carazolol and β₂AR-T4L are depicted schematically in FIG. 6. The carbazole ring system is oriented roughly perpendicular to the plane of the membrane, and the alkylamine chain (atoms 15-22 in the model) is nearly parallel to the heterocycle (FIG. 7A-B). Carazolol was modeled into the electron density (3) as the (S)-(−) isomer due to the higher affinity of this enantiomer, despite the fact that a racemic mixture of the ligand was used in crystallization. Asp113^(3.32), Tyr316^(7.43), and Asn312^(7.39) present a constellation of polar functional groups to the alkylamine and alcohol moieties of the ligand, with Asp113^(3.32) and Asn312^(7.39) sidechains forming close contacts (<3 Å) with O₁₇ and N₁₉ atoms of carazolol (FIGS. 6 and 7A-B). Asp113^(3.32) was one of the first β₂AR residues shown to be important for ligand binding; notably the D113N mutation causes complete loss of detectable affinity for antagonists and a decrease in the potency of agonists towards cell-based G protein activation by over 4 orders of magnitude. Likewise, mutations of Asn312^(7.39) perturb β₂AR binding to agonists and antagonists: changes to nonpolar amino acids (Ala or Phe) reduce affinities to undetectable levels, while retention of a polar functionality (Thr or Gln) gives partial affinity. On the opposite end of the ligand near helix V, N₇ of the carbazole heterocycle forms a hydrogen bond with the side chain hydroxyl of Ser203^(5.42). Interestingly, mutations of Ser203^(5.42) specifically decrease β₂AR affinity towards catecholamine agonists and aryloxyalkylamine ligands with nitrogen-containing heterocycles such as pindolol, and by implication carazolol. Thus, the polar interactions between carazolol and the receptor observed in the crystal structure agree with the known biochemical data. The contribution of Tyr316^(7.43) to antagonist and agonist affinity remains to be tested; this residue is conserved as tyrosine in all sequenced adrenergic receptor genes.

FIG. 7C shows the tight packing between carazolol and surrounding amino acids that buries 790 Å² of surface area from solvent; specific contacts are depicted schematically in FIG. 6. Notable among the hydrophobic residues contacting carazolol are Val114^(3.33), Phe290^(6.52), and Phe193^(5.32). The side chain of Val114^(3.33) from helix III makes multiple contacts with the C₈-C₁₃ ring of the carbazole heterocycle, and Phe290^(6.52) from helix VI forms an edge-to-face aromatic interaction with the same ring. As a result, these two amino acids form a hydrophobic “sandwich” with the portion of the aryl moiety that is common to many adrenergic antagonists. Mutation of Val114^(3.33) to alanine was shown to decrease β₂AR affinity towards the antagonist alprenolol by an order of magnitude, as well as lowering affinity for the agonist epinephrine 300-fold. Phe193^(5.32) is different from other carazolol contact residues in that it is located on the ECL2, in the path of hormone accessibility to the binding pocket. This amino acid contributes more buried surface area than any other residue to the interface between β₂AR-T4L and carazolol (see Table S3). Therefore, Phe193^(5.32) is likely to contribute significantly to the energy of β₂AR-carazolol complex formation, and the position of this residue on the extracellular side of the binding site may allow it to act as a gate that contributes to the unusually slow dissociation of the ligand (FIG. 14).

Analysis of the binding pocket provides insights into the structural basis for pharmacologic selectivity between the β₂AR and closely related adrenergic receptors such as the β₁AR. The affinities of these two receptors for certain ligands, such as ICI-118,551, betaxolol and RO363, differ by up to 100-fold. Curiously, all of the amino acids in the carazolol binding pocket are conserved between the β₁AR and β₂AR (see FIG. 15). The majority of the 94 amino acid differences between the β₁AR and β₂AR are found in the cytoplasmic and extracellular loops. While residues that differ in the transmembrane segments generally face the lipid bilayer, eight residues lie at the interface between helices and may influence helix packing. The structural basis for pharmacologic differences between β₁AR and β₂AR must, therefore, arise from amino acid differences in the entrance to the binding pocket or subtle differences in the packing of helices. Evidence for the latter comes from chimeric receptor studies in which successive exchange of helices between β₁AR and β₂ARs led to a gradual change in affinity for the β₂AR selective ICI-118,551 and the β₁AR selective betaxolol.

As discussed above, β₂AR-T4L shows CAM-like properties with respect to agonist binding affinities, suggesting that the unliganded β₂AR-T4L may exist in a more active conformation than the wild type-β₂AR. Nevertheless, as shown in FIG. 4B, β₂AR-T4L can be stabilized in an inactive conformation by an inverse agonist. Since β₂AR-T4L was crystallized with bound carazolol, a partial inverse agonist, the structure most likely represents an inactive state. This is consistent with the similarity of the β₂AR-T4L and β₂AR-Fab5 carazolol-bound structures. To assess whether conformational changes are required to accommodate catecholamines, a model of isoproterenol was placed in the binding site such that common atoms (16-22 in FIG. 6) were superimposed onto the analogous carazolol coordinates in the crystal structure (FIG. 7D). Residues Ser204^(5.43) and Ser207^(5.46) are critical for catecholamine binding and activation of the β₂AR, with Ser204^(5.43) hydrogen bonding to the meta-hydroxyl and Ser207^(5.46) to the para-hydroxyl of the catechol ring, respectively. In our model, the catechol hydroxyls of isoproterenol face the appropriate serines on helix V, but the distances are too long for hydrogen bonding (6.8 Å from meta-hydroxyl oxygen to the sidechain oxygen of Ser204^(5.43), 4.8 Å from the para-hydroxyl oxygen to the sidechain oxygen of Ser207^(5.46)). In addition, Asn293^(6.55) and Tyr308^(7.35), two residues expected to form selective interactions with agonists based on the literature, are too distant to form productive polar or hydrophobic contacts with the modeled isoproteronol molecule. These observations suggest that agonist binding requires changes in the binding site relative to the carazolol-bound structure, unless common structural components of agonists and inverse agonists bind in a significantly different manner.

EXAMPLE 6 Structural Insights into β₂AR Activation

Biophysical studies provide evidence that conformational changes associated with activation of the β2AR are similar to those observed for rhodopsin. Yet the highly efficient process of light activation of rhodopsin through the cis-trans isomerization of covalently bound retinal is very different from activation of the β₂AR and other GPCRs by diffusible hormones and neurotransmitters. Despite representing a static picture of the inverse agonist-bound state, the crystal structure of β₂AR-T4L still shows how agonist binding is translated into structural changes in the cytoplasmic domains of receptor. Agonist binding occurs at the extracellular ends of helices III, IV, V and VII, and G protein activation is mediated by the cytoplasmic ends. While the structure is open at the extracellular face to form the ligand binding pocket, the helices are more closely packed in the intracellular half of the receptor. This close packing implies that isolated rigid-body movement of any of these helices is unlikely, and that conformational changes can only be accomplished by rearrangement of side chains forming the network of interactions between the helices. Biophysical studies show that structurally different agonists stabilize distinct active states, suggesting that different ligands could stabilize different combinations of side chain rearrangements.

Analysis of mutations that affect β₂AR function provides insights into structural rearrangements that are likely to occur during receptor activation. FIG. 8A illustrates the location of amino acids for which mutations lead to elevated basal, agonist-independent activity (constitutively active mutations, CAMs), as well as amino acids for which mutations impair agonist activation (uncoupling mutations, UCMs). Residues for which CAMs have been described are likely to be involved in interactions that maintain the receptor in the inactive conformation. These amino acids are centrally located on helices III and VI. In contrast, positions in which UCMs have been observed are likely to form intramolecular interactions that stabilize the active state. A cluster of UCMs are found at the cytoplasmic end of helix VII. Neither CAMs nor UCMs are directly involved in agonist binding. Although the CAMs and UCMs are not directly connected in sequence, it is evident from the structure that they are linked through packing interactions, such that movements in one will likely affect the packing of others. For example, FIG. 8A (right panel) shows all amino acids with atoms within 4 Å of the two centrally located CAMs, Leu124^(3.43) and Leu272^(6.34). Several amino acids that pack against these CAMs also interact with one or more UCMs. Trp286^(6.48) lies at the base of the binding pocket. It has been proposed that agonist binding leads to a change in the rotameric state of Trp286^(6.48) with subsequent changes in the angle of the helical kink formed by Pro288^(6.50). It is likely that an agonist-induced change in the rotameric state of Trp286^(6.48) will be linked to changes in sidechains of CAMs and UCMs through packing interactions and propagated to the cytoplasmic ends of the helices and the associated intracellular loops that interact with G proteins and other signaling molecules.

In the structures of both rhodopsin and the β₂AR, a cluster of water molecules lies near the most highly conserved class A GPCR residues (FIG. 8B). It has been proposed that these water molecules may play a role in the structural changes involved in receptor activation. FIG. 8C shows the network of potential hydrogen bonding interactions that link Trp286^(6.48) with conserved amino acids extending to the cytoplasmic ends of helices. UCMs have been identified for three amino acids linked by this network—N322^(7.49), P323^(7.50), and Y326^(7.53). This relatively loose-packed, water filled region is likely to be important in allowing conformational transitions, as there will be fewer steric restraints to sidechain repacking. Future structures of the agonist-bound state of the β₂AR will help to clarify the precise rearrangements that accompany activation of the receptor. 

1. An isolated GPCR fusion protein comprising, from N-terminus to C-terminus: i. a first portion of a G-protein coupled receptor (GPCR), wherein said first portion comprises TM1, TM2, TM3, TM4 and TM5 regions of said GPCR; ii. a domain comprising the amino acid sequence of a lysozyme; and iii. a second portion of said GPCR, wherein said second portion comprises TM6 and TM7 regions of said GPCR.
 2. The isolated GPCR fusion protein of claim 1, wherein said isolated GPCR fusion protein is present in an isolated membrane.
 3. The isolated GPCR fusion protein of claim 1, wherein said isolated GPCR fusion protein is present on the surface of a cell.
 4. The isolated GPCR fusion protein of claim 1, wherein said isolated GPCR fusion protein is epitope tagged.
 5. The isolated GPCR fusion protein of claim 1, wherein said isolated GPCR fusion protein is active.
 6. The isolated GPCR fusion protein of claim 1, wherein said domain comprises an amino acid sequence having at least 80% identity to the amino acid sequence of a wild-type lysozyme.
 7. The isolated GPCR fusion protein of claim 1, wherein said domain comprises an amino acid sequence having at least 95% identity to a wild-type lysozyme.
 8. The isolated GPCR fusion protein of claim 1, wherein said domain comprises an amino acid sequence that is at least 95% identical to the amino acid sequence of T4 lysozyme.
 9. The isolated GPCR fusion protein of claim 1, wherein said first and second portions of said GPCR comprise the amino acid sequence of a naturally occurring GPCR.
 10. The isolated GPCR fusion protein of claim 1, wherein said first and second portions of said GPCR comprise the amino acid sequence of a non-naturally occurring GPCR.
 11. The isolated GPCR fusion protein of claim 1, wherein the amino acid sequence of said first and second portions of said GPCR are least 80% identical to amino acid sequences of a wild type GPCR.
 12. The isolated GPCR fusion protein acid of claim 1, wherein said a GPCR is selected from the group consisting of: a receptor for a biogenic amine, a dopamine receptor, a serotonin receptor, an adrenergic receptor, a β2-adrenergic receptor, a melanocortin receptor subtype 4, a ghrelin receptor, a metabotropic glutamate receptor and a chemokine receptor.
 13. The isolated GPCR fusion protein of claim 1, wherein said domain is in the range of from 100 to 200 amino acids in length.
 14. A polypeptide comprising, from N-terminus to C-terminus: a) a first portion of a G-protein coupled receptor (GPCR), wherein said first portion comprises the amino acid sequence that is N-terminal to the IC3 loop of said GPCR; b) a domain comprising the amino acid sequence of a lysozyme; c) a second portion of said GPCR, wherein said second portion comprises the amino acid sequence that is C-terminal to the IC3 loop of said GPCR.
 15. The polypeptide of claim 14, wherein said domain spaces the C-terminal end of the first portion of said GPCR and the N-terminal end of the second region of said GPCR so that the closest alpha carbon atoms at said C-terminal end and said N-terminal end are spaced by a distance of in the range of 6 Å to 16 Å.
 16. An isolated G-protein coupled receptor (GPCR) comprising an IC3 loop containing a substitution that comprises the amino acid sequence of a lysozyme. 